How to fix this sentence fragment. Richard III supposedly had the young princes murdered. Found out what really happened to them.

I would make it into a compound sentence. Put a semicolon after murdered; and add "no one has" before the found.....etc.

To fix the sentence fragment and turn it into a compound sentence, you can follow these steps:

1. Identify the fragments: "Found out what really happened to them."
2. Combine the fragments with the previous sentence: Richard III supposedly had the young princes murdered; then, found out what really happened to them.

By placing a semicolon after "murdered," you signal that the two parts are closely related. Also, you can add "then" to indicate the sequence of actions. Here's the revised version:

"Richard III supposedly had the young princes murdered; then, he found out what really happened to them."

Now the sentence is a compound sentence that connects the related thoughts in a concise manner.